1. Field of the Invention
This invention relates generally to a method for controlling the speed of a vehicle traveling downhill that has a regenerative braking capability and, more particularly, to a method for controlling the speed of a vehicle traveling downhill that has a regenerative braking capability by using a cruise control system of the vehicle.
2. Discussion of the Related Art
Fuel cell, battery electric and hybrid vehicles employ a supplemental power source, such as a high voltage DC battery, an ultracapacitor and/or a flywheel. The supplemental power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the main power source is unable to provide the desired power. A fuel cell stack or an engine with a generator provides power to an electric traction system through a DC high voltage electrical bus for vehicle operation. The battery or ultracapacitor provides supplemental power to the electrical bus during those times when additional power is needed beyond what the continuous electric power source can provide, such as during heavy acceleration. For example, a fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW of power.
The regenerative power available from an electric traction system during regenerative braking is also used to recharge the battery or ultracapacitor. The ancillary loads of the vehicle and in the case of hybrids and fuel cell vehicles, the engine or fuel cell system, may act as an additional energy sink in case the battery is fully charged. The brake system and the propulsion system may have a brake blending function, which will distribute on driver brake pedal demand the available regenerative energy in an optimum of efficiency and system reliability between the brakes, the battery and the electric loads. Besides alternative propulsion systems, the brake system will have more and more active functions, and if such an active brake in a conventional engine driven vehicle allows brake control without applying the brake pedal, the functions of inversed cruise control can apply also.
An electric vehicle typically is able to provide regenerative braking where the electric traction motor on the vehicle operates as a generator that generates current in response to a driving force on the motor. The regenerative braking energy can be used to recharge the battery or ultracapacitor, or operate system loads.
When a vehicle is traveling downhill, it will typically gain speed. In order to maintain a constant speed and prevent the vehicle from increasing its speed, the vehicle operator may have to operate the vehicle's brakes to prevent the increase in speed. For a conventional vehicle, the vehicle operator can put the vehicle into a lower gear to allow the engine to prevent or reduce an increase in the vehicle speed. Using the brake system has disadvantages because of over-dimensioning the brakes and/or providing additional wear and tear on the brakes. For an electric vehicle, lower gears may not be provided or the electric drive motor may not have an intrinsic characteristic of braking torque. Thus, putting the vehicle in a lower gear to reduce the vehicle speed when traveling downhill may not be an option.